Series III fuel gauge

Does anyone know the voltage that the Series III fuel sender should be
getting from the regulator?

I'm running a Lightweight, and as far as I can tell the actual sender is
the same. The voltage applied I measure at about 4.5 volts on the
disconnected lead, which seems low to me. The manual changeover switch
seems to be working.

This would suggest the fault is further up the chain, either the actual
gauge or the voltage regulator (it's a 12-volt system).




--
David G. Bell -- SF Fan, Filker, and Punslinger.

"I am Number Two," said Penfold. "You are Number Six."

""David G. Bell"" <[email protected]> wrote in message
news:[email protected]...
> Does anyone know the voltage that the Series III fuel sender should be
> getting from the regulator?
>
> I'm running a Lightweight, and as far as I can tell the actual sender is
> the same. The voltage applied I measure at about 4.5 volts on the
> disconnected lead, which seems low to me. The manual changeover switch
> seems to be working.
>
> This would suggest the fault is further up the chain, either the actual
> gauge or the voltage regulator (it's a 12-volt system).
>

I assume your fuel gauge isn't working from this. Are you trying to measure
the output from the fuel gauge supply regulator or the lead to the tank
unit? The regulator (on the back of the speedometer) keeps the supply
constant so the gauge reads the same as the battery voltage fluctuates ( e.g
charging after start up) and supplies a regulated 12 volts but the voltage
(and current ) to the tank unit are very small ( I don't think you really
want to put 12 volts in the tank for obvious reasons) and any dirty
connections etc. will consequently affect the reading of the gauge. Check if
the gauge moves when earthing the tank unit lead to the chassis as the most
common fault seems to be corrosion at the tank mountings and not completing
the circuit.

HTH Martin

On Friday, in article
<[email protected]>
[email protected] "Oily" wrote:

> ""David G. Bell"" <[email protected]> wrote in message
> news:[email protected]...
> > Does anyone know the voltage that the Series III fuel sender should be
> > getting from the regulator?
> >
> > I'm running a Lightweight, and as far as I can tell the actual sender is
> > the same. The voltage applied I measure at about 4.5 volts on the
> > disconnected lead, which seems low to me. The manual changeover switch
> > seems to be working.
> >
> > This would suggest the fault is further up the chain, either the actual
> > gauge or the voltage regulator (it's a 12-volt system).
> >
>
> I assume your fuel gauge isn't working from this. Are you trying to measure
> the output from the fuel gauge supply regulator or the lead to the tank
> unit? The regulator (on the back of the speedometer) keeps the supply
> constant so the gauge reads the same as the battery voltage fluctuates ( e.g
> charging after start up) and supplies a regulated 12 volts but the voltage
> (and current ) to the tank unit are very small ( I don't think you really
> want to put 12 volts in the tank for obvious reasons) and any dirty
> connections etc. will consequently affect the reading of the gauge. Check if
> the gauge moves when earthing the tank unit lead to the chassis as the most
> common fault seems to be corrosion at the tank mountings and not completing
> the circuit.

Thanks. Yes, I was reading from the lead to the tank sender, while
disconnected from the sender. And the other meter lead to a good contact
on the chassis. Bother, don't have an impedance figure for the meter to
hand, but 4.5v certainly implies a pretty huge resistance for the gauge
and wiring downstream of the regulator, dropping 7.5v -- I rather doubt
that a 1960s fuel gauge would have a useful response to currents
measured in microamperes.

I'll check tank-to-chassis. Since I'm getting a voltage, the fuse hasn't
blown, but check the contacts there...



--
David G. Bell -- SF Fan, Filker, and Punslinger.

"I am Number Two," said Penfold. "You are Number Six."

""David G. Bell"" <[email protected]> wrote in message
news:[email protected]...
> On Friday, in article
> <[email protected]>
> [email protected] "Oily" wrote:
>
> > ""David G. Bell"" <[email protected]> wrote in message
> > news:[email protected]...
> > > Does anyone know the voltage that the Series III fuel sender should be
> > > getting from the regulator?
> > >
> > > I'm running a Lightweight, and as far as I can tell the actual sender
is
> > > the same. The voltage applied I measure at about 4.5 volts on the
> > > disconnected lead, which seems low to me. The manual changeover switch
> > > seems to be working.
> > >
> > > This would suggest the fault is further up the chain, either the
actual
> > > gauge or the voltage regulator (it's a 12-volt system).
> > >
> >
> > I assume your fuel gauge isn't working from this. Are you trying to
measure
> > the output from the fuel gauge supply regulator or the lead to the tank
> > unit? The regulator (on the back of the speedometer) keeps the supply
> > constant so the gauge reads the same as the battery voltage fluctuates
( e.g
> > charging after start up) and supplies a regulated 12 volts but the
voltage
> > (and current ) to the tank unit are very small ( I don't think you
really
> > want to put 12 volts in the tank for obvious reasons) and any dirty
> > connections etc. will consequently affect the reading of the gauge.
Check if
> > the gauge moves when earthing the tank unit lead to the chassis as the
most
> > common fault seems to be corrosion at the tank mountings and not
completing
> > the circuit.
>

> Bother, don't have an impedance figure for the meter to
> hand, but 4.5v certainly implies a pretty huge resistance for the gauge

Yes it has. It's a wire wound coil round a bi-metallic strip which bends
and moves the needle when heated by current passing through it on series 3
and the '60s one is magnetic deflection with a variable resistor in the tank
unit which wears through the coil in time. If you earth the lead at the tank
with the '60s one it will read empty instantly but the series 3 one will go
to full slowly. You said series 3 *and* '60s so I don't know which one you
have got.

> a 1960s fuel gauge would have a useful response to currents
> measured in microamperes.
>

Yes, they do I think, but I've never needed to measure them. But with the
reading you have it appears that all is well down to the tank and it will be
the tank unit itself which is faulty or just a bad earth on the tank.

Martin

"Oily" <[email protected]> wrote in message
news:[email protected]...
> > I assume your fuel gauge isn't working from this. Are you trying to
measure
> the output from the fuel gauge supply regulator or the lead to the tank
> unit? The regulator (on the back of the speedometer) keeps the supply
> constant so the gauge reads the same as the battery voltage fluctuates (
e.g
> charging after start up) and supplies a regulated 12 volts but the voltage
> (and current ) to the tank unit are very small ( I don't think you really
> want to put 12 volts in the tank for obvious reasons) and any dirty
> connections etc. will consequently affect the reading of the gauge. Check
if
> the gauge moves when earthing the tank unit lead to the chassis as the
most
> common fault seems to be corrosion at the tank mountings and not
completing
> the circuit.
>
> HTH Martin

I echo what "Oily" says about the earthing. Thoroughly clean up the top of
the sender unit and the 6 (?) little bolts that attach it to the tank. My
gauge was under-recording for 2 or 3 years then finally ceased to register
at all. Cleaning things as above got the gauge working fine again.

Dave
1983 series 3 SWB 2.25 petrol

On Friday, in article <[email protected]>
[email protected] "David Bexhall" wrote:

> "Oily" <[email protected]> wrote in message
> news:[email protected]...
> > > I assume your fuel gauge isn't working from this. Are you trying to
> measure
> > the output from the fuel gauge supply regulator or the lead to the tank
> > unit? The regulator (on the back of the speedometer) keeps the supply
> > constant so the gauge reads the same as the battery voltage fluctuates (
> e.g
> > charging after start up) and supplies a regulated 12 volts but the voltage
> > (and current ) to the tank unit are very small ( I don't think you really
> > want to put 12 volts in the tank for obvious reasons) and any dirty
> > connections etc. will consequently affect the reading of the gauge. Check
> if
> > the gauge moves when earthing the tank unit lead to the chassis as the
> most
> > common fault seems to be corrosion at the tank mountings and not
> completing
> > the circuit.
> >
> > HTH Martin
>
> I echo what "Oily" says about the earthing. Thoroughly clean up the top of
> the sender unit and the 6 (?) little bolts that attach it to the tank. My
> gauge was under-recording for 2 or 3 years then finally ceased to register
> at all. Cleaning things as above got the gauge working fine again.

Yes, getting that done fixed things.



--
David G. Bell -- SF Fan, Filker, and Punslinger.

"I am Number Two," said Penfold. "You are Number Six."